A liquid crystal display is a thin, light-weight, low power display (image display) apparatus as compared with a Braun tube. Therefore, the liquid crystal display has been used in wide ranging applications including image display apparatuses such as televisions and videos, office automation (OA) equipment such as monitors, word processors and personal computers, and the display portions of cellular phones and portable terminals.
As a liquid crystal display, e.g., a twisted nematic (TN) mode liquid crystal display has been put into practical use. However, it has the disadvantages of slow response and a narrow viewing angle. Ferroelectric liquid crystals (FLC) or antiferroelectric liquid crystals (AFLC), which can provide quick response and a wide viewing angle, also have been proposed. However, there are serious disadvantages in shock resistance and temperature characteristics, and so far they have not been used widely. A display mode using polymer-dispersed liquid crystals that utilizes light scattering does not require a polarizing plate and can provide high brightness. However, it cannot control the viewing angle inherently with a phase difference plate and also has a problem of response characteristics at the moment. Thus, the polymer-dispersed liquid crystals are less superior to the TN mode liquid crystals.
In recent years, particularly with a rapid increase in information processing speed in the application field of cellular phones and portable displays, there have been demands for a motion display function in the market. To meet this demand, the OCB mode is proposed as a display mode that can achieve quick response and a wide viewing angle. The R-OCB mode also is proposed as a result of application of the OCB mode to a reflection-type mode.
FIG. 16 is a cross-sectional view showing the configuration of an OCB mode liquid crystal display. This liquid crystal display includes a substrate 401 provided with a transparent electrode 402, a substrate 408 provided with a transparent electrode 407, and a liquid crystal layer 404 sandwiched between the substrates 401, 408. Alignment films 403, 406 are formed on the transparent electrodes 402, 407, respectively. The alignment films 403, 406 are processed so that liquid crystal molecules orient parallel to one another in the same direction. Polarizing plates 413, 416 are arranged on the outer sides of the substrates 401, 408 so that their polarization axes are orthogonal to each other. Phase compensation plates 417, 418 are interposed between the polarizing plate 413 and the substrate 401 and between the polarizing plate 416 and the substrate 408, respectively.
In the OCB mode liquid crystal display, when the liquid crystal layer is in the initial state (i.e., no voltage is applied), it has a splay orientation 4a. The application of a voltage induces a bend orientation 4b or a bend orientation including a twisted orientation in the liquid crystal layer, and display is performed in this bend orientation state.
However, it takes a long time for a transition from the splay orientation to the bend orientation in a conventional OCB mode liquid crystal display, which has been a major obstacle to its practical use. The transition is performed generally by applying a high voltage across the opposed electrodes. Though the transition time becomes shorter with increasing voltage, a higher voltage cannot be applied readily because of the withstand voltage of an IC driving voltage. Therefore, the transition should be performed while applying a voltage of about several to 30 V. With the application of several volts, however, a period of time in minutes is required to complete the transition.
FIG. 23 is a cross-sectional view showing the configuration of a R-OCB mode liquid crystal display. This liquid crystal display includes a substrate 501 provided with a transparent electrode 502, a substrate 508 provided with a transparent electrode 507, and a liquid crystal layer 504 sandwiched between the substrates 501, 508. Alignment films 503, 506 are formed on the transparent electrodes 502, 507, respectively. A horizontal alignment film is used as the alignment film 503, and a vertical alignment film is used as the alignment film 506. Polarizing plates 513, 516 are arranged on the outer sides of the substrates 501, 508 so that their polarization axes are orthogonal to each other. Phase compensation plates 517, 518 are interposed between the polarizing plate 513 and the substrate 501 and between the polarizing plate 516 and the substrate 508, respectively. A reflecting plate (not shown) is arranged on the inner or outer surface of the substrate 508.
In the R-OCB mode liquid crystal display, when the liquid crystal layer is in the initial state (i.e., no voltage is applied), the liquid crystal molecules on one substrate side orient perpendicular to the substrate surface and those on the other substrate side orient parallel to the substrate surface. For display, the liquid crystal molecules in the central portion of the liquid crystal layer should be controlled so as to align perpendicular to the substrates by applying a voltage. However, unlike the OCB mode, the transition process is not necessary because the pretilt angle of the liquid crystal molecules at one substrate differs from that of the liquid crystal molecules at the other substrate.
As described above, the orientations of the liquid crystal molecules at the two substrates should differ in a conventional R-OCB mode liquid crystal display. Therefore, different types of alignment films are used for the respective substrates: the horizontal alignment film as the alignment film 3 and the vertical alignment film as the alignment film 6. Consequently, electrical asymmetry occurs between the substrates. This leads to nonuniform display density and a residual image caused when the same image is displayed for a long time even under the normal operating conditions. Thus, the display quality is degraded considerably.
It is an object of the present invention to achieve a quick transition from the splay orientation to the bend orientation by applying a relatively low voltage in an OCB mode liquid crystal display.
It is another object of the present invention to reduce electrical asymmetry between the substrates and improve the display quality of a liquid crystal display in which the pretilt angle at one substrate differs from that at the other substrate, such as a R-OCB mode liquid crystal display.